[PowerPC] More precise exploitation of P9 maddld instruction when operands are constant

There are 3 operands of maddld, (add (mul %1, %2), %3) and sometimes
they are constant. If there is constant operand, it takes extra li to 
materialize the operand, and one more extra register too. So it's not 
profitable to use maddld to optimize mul-add pattern.

Differential Revision: https://reviews.llvm.org/D60181

llvm-svn: 358253